An applied study on an organic EL device as a luminescence device of a high-speed responsiveness and a high efficiency has been energetically conducted. Basic structures thereof are shown in FIGS. 1(a) and (b) (e.g., Macromol. Symp. 125, 1–48 (1997)).
As shown in FIG. 1, an organic EL device generally has a structure comprising a transparent electrode 14, a metal electrode 11, and a plurality of organic film layers therebetween on a transparent substrate 15.
In the device of FIG. 1(a), the organic layers comprise a luminescence layer 12 and a hole-transporting layer 13. For the transparent electrode 14, ITO, etc., having a large work function are used, for providing a good hole-injection characteristic from the transparent electrode 14 to the hole-transporting layer 13. For the metal electrode 11, a metal, such as aluminum, magnesium or an alloy of these, having a small work function is used for providing a good electron-injection characteristic to the organic layers. These electrodes have a thickness of 50–200 nm.
For the luminescence layer 12, aluminum quinolinol complexes (a representative example thereof is Alq3 shown hereinafter), etc., having an electron-transporting characteristic and luminescence characteristic are used. For the hole-transporting layer 13, biphenyldiamine derivatives (a representative example thereof is α-NPD shown hereinafter), etc., having an electron-donative characteristic are used.
The above-structured device has a rectifying characteristic, and when an electric field is applied between the metal electrode 11 as a cathode and the transparent electrode 14 as an anode, electrons are injected from the metal electrode 11 into the luminescence layer 12 and holes are injected from the transparent electrode 15. The injected holes and electrons are recombined within the luminescence layer 12 to form excitons and cause luminescence. At this time, the hole-transporting layer 13 functions as an electron-blocking layer to increase the recombination efficiency at a boundary between the luminescence layer 12 and hole-transporting layer 13, thereby increasing the luminescence efficiency.
Further, in the structure of FIG. 1(b), an electron-transporting layer 16 is disposed between the metal electrode 11 and the luminescence layer 12. By separating the luminescence and the electron and hole-transportation to provide a more effective carrier blocking structure, efficient luminescence can be performed. For the electron-transporting layer 16, an electron-transporting material, such as an oxadiazole derivative, can be used.
Luminescence used heretofore in organic EL devices generally includes two types including fluorescence and phosphorescence. In a fluorescence device, fluorescence at the time of transition of luminescence material molecule from a singlet exciton state to the ground state is produced. On the other hand, in a phosphorescence device, luminescence via a triplet exciton state is utilized.
In recent years, the phosphorescence device providing a higher luminescence yield than the fluorescence device has been studied.
Representative published literature may include:                Article 1: Improved energy transfer in electrophosphorescent device (D. F. O'Brien, et al., Applied Physics Letters, Vol. 74, No. 3, p. 422 (1999)); and        Article 2: Very high-efficiency green organic light-emitting devices based on electrophosphorescence (M. A. Baldo, et al., Applied Physics Letters, Vol. 75, No. 1, p. 4 (1999)).        
In these articles, a structure including 4 organic layers as shown in FIG. 1(c) has been principally used, including, from the anode side, a hole-transporting layer 13, a luminescence layer 12, an exciton diffusion-prevention layer 17 and an electron-transporting layer 16. Materials used therein include carrier-transporting materials and phosphorescent materials. Abbreviations of the respective materials are as follows.                Alq3: aluminum quinolinol complex        α-NPD: N4,N4′-di-naphthalene-1-yl-N4,N4′-diphenyl-biphenyl-4,4′-diamine        CBP: 4,4′-N,N′-dicarbazole-biphenyl        BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline        PtOEP: platinum-octaethylporphyrin complex        Ir(ppy)3: iridium-phenylpyridine complex        

However, the organic EL device utilizing phosphorescence described above is accompanied with a problem regarding a deterioration in luminescence particularly in an energization state. The reason of the deterioration has not been clarified, but is conceived as follows. Generally, a life of the triplet excitons is longer by three or more digits than the life of a singlet exciton, so that excited molecules are held in a high-energy state for a longer period. As a result, it may be considered that reaction with surrounding materials such as polymer formation among the excitons, a change in minute molecular structure and a change in structure of the surrounding material are caused.
Anyway, the phosphorescence device is expected to have a high luminescence efficiency but on the other hand, the device is problematic in terms of deterioration in energized state. As a result, the luminescent material used in the phosphorescence device is desired to be a compound providing a high-efficiency luminescence and a high stability.